The pacemaker currents and their control will be studied in cardiac Purkinje and sino-atrial node myocytes. In Purkinje fibers, the pacemaker current is generally believed to be If. Because several findings are inconsistent with this interpretation, we studied this problem in single Purkinje myocytes by means of a whole cell patch clamp technique (Vassalle et al., 1994,1995). We found that in the pacemaker range of potentials a time-dependent current (Ikdd) flows that reverses at Ek (and such reversal shifts in different [K]0 according to the calculated Ek), undergoes a decrease in slope conductance, is blocked by Cs+ and is decreased by high [K]0. In the presence of Ba2+, Ikdd is blocked and another time-dependent current (If) is unmasked that has a threshold generally negative to Ek, does not reverse at more negative potentials, undergoes an increase in slope conductance and is increased by high [K]0. Thus, the pacemaker current due to the decay of a K+ conductance Ikdd in the voltage range of diastolic depolarization can be separated from a hyperpolarization-activated current If at more negative potentials. It will now be possible to separately investigate in the same cell these two currents, their physiological role in diastolic depolarization and their control. The experiments will be conducted in single Purkinje cells by using whole voltage clamp technique in the absence and presence of Ba2+. The aims include the analysis of the possible contribution of Ikl to the reversed Ikdd, the reasons why If does not appear to interfere with the reversal of Ikdd in the absence of Ba2+, the possible significance of If in terms of automaticity, the kinetics of Ikdd uncontaminated by If and by fluctuations in [K+]0, and mechanisms of some controlling factors. In Purkinje strands, the reversal of the pacemaker current is often more negative than expected and is shifted by several procedures, suggesting either fluctuations of K+ in narrow spaces or mixed reversal due to If activation. The pacemaker current in the sino-atrial node (SAN) is also controversial, as it has been attributed either to the decay of Ik or to the activation of If. We found that even 20 mM Cs+ fails to suppress the SAN discharge, although 2 mM Cs+ suppress If in single SAN myocytes. The contribution of diastolic time-dependent currents to the pacemaker potential will be analyzed in single SAN myocytes by suitable dissecting procedures. Thus, a fresh appraisal of Purkinje and SAN automaticity is needed by studying pacemaker mechanisms in Purkinje and sino-atrial node myocytes with methods that allow their separate analysis as described here.